The process for companies to prepare bids to build and operate a 16-mile Purple Line in Montgomery and Prince George’s counties formally began Tuesday, when the Maryland transportation department released a 2,200-page bid solicitation for the light-rail project.

Four teams of private companies will have until Jan. 9 to submit their proposals for a 35-year public-private partnership in which the companies would design, build, operate, maintain and help finance a Purple Line’s construction. The state expects the winning team to contribute $500 million to $900 million toward the project’s estimated $2.37 billion construction costs.

A preferred bid will be chosen by spring 2015, officials said. If the contract is approved by the Maryland Board of Public Works, construction would start later next year, officials said. The line would open in 2020, they said.

A Purple Line would connect Bethesda in Montgomery with New Carrollton in Prince George’s. The line, which would run inside the Capital Beltway, would have 21 stations, including in North Chevy Chase, Silver Spring, Langley Park, College Park and Riverdale Park. It would connect Maryland’s Metrorail lines, and supporters say it would improve east-west transit while spurring redevelopment around stations.

Opponents say the project is too expensive and would cause too much environmental damage, including to an endangered shrimp-like animal and the wooded extension of the Capital Crescent Trail between Bethesda and Silver Spring.

While the Maryland Department of Transportation released the request for proposals Tuesday, companies already had begun preparing their bids. The four companies were selected to bid in January.

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